The present invention relates to an atomic layer epitaxy method and an apparatus therefor and, more particularly, to a compound semiconductor atomic layer epitaxy method using molecules having a self-limiting mechanism as one source material and an apparatus therefor.
In a conventional atomic layer epitaxy method, in order to grow GaAs crystals, for example, trimethylgallium [TMG, (CH.sub.3).sub.3 Ga] as an organometal gas and arsine [ASH.sub.3 ] are used as a Ga source material and an As source material, respectively, and alternately supplied on a substrate crystal. In this atomic layer epitaxy method, when TMG is supplied, a part of an alkyl group of TMG is chemically removed and chemically bonded to As on the substrate surface to form one Ga layer on the As substrate crystal. Since TMG which subsequently flies onto the formed layer is not adsorbed, the growth is stopped when one Ga monolayer is formed. This mechanism is called a self-limiting mechanism, and atomic layer epitaxy is achieved by using this mechanism. Such a conventional atomic layer epitaxy method is performed at a comparatively low substrate temperature (about 500.degree. C.) under the condition of a very narrow growth temperature margin.
This is because if all chemical bondings of three alkyl groups and Ga atoms are thermally dissociated to form atomic Ga in a vapor phase before TMG's reaching the substrate surface, Ga adheres on Ga resulting in growth in units of atomic layers. That is, self-limiting mechanism is damaged in the atomic Ga. Consequently, in the conventional method, the upper limit of the growth temperature is defined by a temperature Th at which thermal dissociation of the source material in vapor phase occurs. On the other hand, if the growth temperature is too low, the source material gas cannot be chemically bonded to atoms on the substrate surface leading to fail in growth of an atomic layer. Therefore, the lower limit of the growth temperature is defined by a temperature Tl at which the source material is chemically bonded with the substrate surface atoms and grown (bonded) by one atomic layer. The values of Th and Tl depend on the type of a source material gas. In GaAs growth using TMG, for example, Th is about 500.degree. C., and Tl is about 490.degree. C. That is, the difference between the two temperatures, i.e., a growth temperature range within which an atomic layer can be grown is only 10.degree. C. This is the reason for the narrow growth temperature margin of the atomic layer epitaxy method.
Conventionally, the growth of GaAs is realized although the growth temperature range is narrow as described above. However, it is difficult to realize a compound semiconductor containing Al atoms such as AlAs or AlGaAs. This is because, although an organometal in which three methyl groups or ethyl groups are bonded to Al metal is normally used, atomic layer epitaxy cannot be performed since no temperature difference is present between Th and Tl. In order to solve this problem, a chloride of Al or a compound in which a part of an alkyl group bonded to Al metal is substituted by chlorine, e.g., diethylaluminumchloride [(CH.sub.3).sub.2 AlCl] is used as a source material. In addition, a method of thermally dissociating the source material to remove a methyl group before supplying the material is also performed. Since AlCl as a product is unstable, however, AlCl chemically reacts with each other to produce atomic Al in a vapor phase and thus self-limiting mechanism is damaged, thereby disabling atomic layer epitaxy. This means that it is impossible to realize atomic layer epitaxy of a hetero structure (e.g., GaAs/AlGaAs or GaAs/AlAs) which is essential in a device structure.
In addition, in the atomic layer epitaxy method, the source material gas contains an alkyl group, and the alkyl group is finally decomposed on the substrate surface, as described above. Therefore, carbon in the alkyl group is mixed as an impurity in crystals. The carbon concentration reaches 10.sup.18 /cm.sup.3 in normal GaAs crystals. Therefore, since it is difficult to realize high-purity crystal growth by this method, the atomic layer epitaxy method is prevented from being put into practical use.